This commit moves the user.Manage to its own package (user/manager)
so it can import the connector package in a later commit.
For clarity, it renames "Manager" to "UserManager" using gorname.
This commit has no functional changes.
When a user attempts to register an email, trim prefixed and
trailing spaces, then perform a basic sanity check to ensure it's
of form "test@example.com".
Fixes#163
